This invention relates to thermal barrier coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating system having a plasma-sprayed bond coat over which a thermal-insulating ceramic layer is deposited, wherein the bond coat undergoes vapor phase aluminizing to have an inward aluminide diffusion that promotes the oxidation resistance of the bond coat while maintaining the as-sprayed surface structure of the bond coat.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, and through the development of oxidation-resistant environmental and thermal barrier coatings deposited on the surface of a superalloy substrate. Environmental coatings are generally employed to protect a superalloy substrate from oxidation, hot corrosion, etc., while thermal barrier coatings further serve to reduce heat transfer to the substrate. As a result, thermal barrier coatings (TBCs) are often used to protect components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor.
To be effective, a thermal barrier coating must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. Coating systems capable of satisfying the above requirements have generally required a metallic bond coat deposited on the component surface, followed by an adherent ceramic layer that serves as the thermal barrier coating. Various ceramic materials have been employed in this role, particularly zirconia (ZrO2) stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), scandia (Sc2O3), or another oxide. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques.
Bond coats of TBC systems are typically formed from an oxidation-resistant aluminum-containing alloy to promote adhesion of the ceramic layer to the component and inhibit oxidation of the underlying superalloy. Examples of bond coats include overlay and diffusion coatings, each of which forms a protective oxide scale during high temperature exposure that chemically bonds the ceramic layer to the bond coat and protects the bond coat and the underlying substrate from oxidation and hot corrosion. Diffusion coatings are formed by reacting the surface of a component with an aluminum-containing composition, which typically yields two distinct zones, an outermost of which is an additive layer that contains the environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone containing various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. The total thickness of a diffusion coating is typically about 50 to 75 micrometers.
Overlay coatings are typically single-layer coatings of MCrAlY, where M is nickel, cobalt, iron or combinations thereof, deposited by low pressure plasma spraying (LPPS) and air plasma spraying (APS). The thickness of an overlay coating is typically about 75 to 175 micrometers. In contrast to diffusion coatings, overlay coatings are not an intermetallic, but are instead metallic solid solutions. Because APS bond coats are deposited at an elevated temperature in the presence of air, they inherently contain oxides and are more prone to oxidation. However, APS bond coats are often favored due to lower equipment cost and ease of application and masking. As a result, various approaches have been proposed to improve the oxidation resistance of APS bond coats, including overcoat aluminiding by which aluminum is diffused into the surface of the bond coat by pack cementation or non-contact vapor (gas) phase techniques. Each of these techniques is similar to that employed to form a diffusion aluminide bond coat, employing a mixture of an aluminum-containing powder (i.e., an aluminum donor), a carrier (activator) such as an ammonium or alkali metal halide, and an inert filler such as alumina to prevent sintering of the powder. The substrate to be treated and the mixture are then heated to about 1200-2200xc2x0 F. (about 650-1200xc2x0 C.) to produce a diffusion aluminide coating.
While overcoat aluminiding by pack cementation methods has been shown to be very effective and practical, as taught by U.S. Pat. No. 5,236,745 to Gupta et al., acceptable results have not been previously achieved with vapor phase deposition techniques because the resulting aluminide is primarily an outward diffusion coating that significantly smooths the surface of the bond coat. Accordingly, while vapor phase aluminiding (VPA) has been employed to form diffusion aluminide environmental coatings and bond coats, VPA processes have not been successful in suitably overcoat-aluminizing a plasma-sprayed bond coat to provide a surface to which a thermal barrier coating will adhere.
The present invention generally provides a thermal barrier coating system and a method for forming the coating system on an article designed for use in a hostile thermal environment, such as superalloy turbine, combustor and augmentor components of a gas turbine engine. The method is particularly directed to a coating system that includes a plasma-sprayed (APS or LPPS) MCrAlY bond coat on which a thermal-insulating APS ceramic layer is deposited, in which the oxidation resistance of the bond coat and the spallation resistance of the ceramic layer are substantially increased by vapor phase aluminizing the bond coat.
According to this invention, the bond coat is deposited to have a surface roughness of at least 300 xcexcinch Ra, preferably about 300 to about 800 xcexcinch Ra, in order to promote the adhesion of the ceramic layer. The oxidation resistance of the bond coat is then substantially improved by overcoat aluminizing the bond coat using a vapor phase process that maintains the as-sprayed surface structure of the bond coat. According to the invention, the surface structure of the bond coat is quantified by the surface area ratio of the bond coatxe2x80x94specifically, the actual surface area of the bond coat (including the slopes of the peaks and valleys) divided by the lateral surface area (the apparent area when viewed in a direction normal to the surface of the bond coat). According to the invention, in order for the vapor phase aluminiding process to maintain the surface area ratio of the bond coat, the process must be carried out at relatively low temperatures that promote inward diffusion of aluminum relative to outward diffusion of the bond coat constituents, particularly nickel and other refractory elements. In addition, process conditions must provide sufficient vapor phase activity at the surface of the bond coat that will promote aluminum atomic movement through the bond coat.
The reliance by this invention on inward diffusion over outward diffusion to overcoat aluminize a TBC bond coat is contrary to diffusion aluminide processes of the prior art, which have provided for outward diffusion to yield thinner diffusions that exhibit enhanced thermal fatigue resistance, as discussed in reference to an aluminide environmental coating disclosed in U.S. Pat. No. 5,217,757. However, in the context of over-aluminiding an APS or LPPS bond coat, the present invention evidences that inward diffusion aluminide layers having a thickness of roughly 75 micrometers can improve the oxidation resistance of the bond coat while having little if any detrimental effect on the surface structure of the bond coat. Accordingly, thermal barrier coating systems formed in accordance with this invention have been shown to exhibit enhanced spallation resistance, in contrast to prior art attempts to overcoat-aluminize bond coats by conventional vapor phase processes, which have resulted in, at best, thermal barrier coatings that rapidly spall when subjected to thermal cycling.